372 W. A. Setchell — Classification and Geographical 



But there are peculiar features, especially in the distribution of the 

 subtribes, that seem to indicate influences of a different nature from 

 any of those mentioned. 



Kjellman says^ that " the causes of the present distribution of the 

 algge in the great divisions of the Ocean need not nor ought to be 

 sought for in the conditions now existing on the earth, any more 

 than the causes of the distribution of the land plants." We then 

 must consider the effect which the various geological changes, 

 especially those of the .more recent periods have wrought upon our 

 kelp-flora. 



Here, however, we are dealing with phenomena which aside from 

 these peculiarities of distribution, have left few data for us. On 

 land we haA'e definite evidences of the changes, other than the vari- 

 ous modifications in the ranges of the land plants or animals. But 

 the changes in the temperature of the surface waters and currents 

 of the sea have to be inferred from such data as we can get from 

 the land. 



The geological changes are more satisfactorily known in the case 

 of the northern hemisphere than of the southern. We know that at 

 one time the northern polar regions were much warmer than they are 

 at present and that there was much freer communication between 

 the polar waters than now exists. We know also that in the Glacial 

 Period, the ice extended down to the coast of France in Europe, to 

 that of New Jersey on the eastern coast of North America and 

 down a considerable distance on the Pacific shores of both North 

 America and Asia. 



These changes, gradually taking place, from the condition of a 

 somewhat open polar sea, to the complete closure of such a sea and 

 the isolation of the marine algae into about four colonies respec- 

 tively situated on the w^est coast of Europe, the northeastern coast 

 of North America, the northwestern coast of North America, and 

 the northeastern coasts of Asia, may perhaps explain many of the 

 present features of distribution of the northern species. Kjellman 

 certainly considers that the glaciation of the northern hemisphere 

 does, for he goes on to say^: "Just as the phanerogamic Flora of 

 Scandinavia contains several elements which are remnants from that 

 period when the glacial formation extended farther southwards than 

 in our days, those arctic algae occurring in the northern Atlantic 

 and the northern Pacific may have stayed there from those times 



^ Arctic Algse, p. 5V. 2 Lq^ qj^^ p 57^ 53. 



